Closed circuit grinding with twostage classification



May 18, 1948. H. MQNEILL CLOSED CIRCUIT GRINDING WITH TWO-STAGECLASSIFICATION Filed June 20, 1942 NEW F550 INVENTOR. HARRY L. M /VEILL359 55 r k 4 m A wk xfikhuko .m mum wow 8 s m M T m m u Patented May 18,1948 CLOSEDfClRQUIT'iGRINDING WITH-TWO- STAGE ICIIASSIFIGA-TIONHarry;L.,McNeill, Denven Colmlip lication lunezo, 1942; SerialNo; 44mm 3Claims.

This-invention relatesgto ore. treatments and more-partixiulairlytdimprovements in ore dress? ing-i'lmethodsz In theearlydaysofpur-metal"production, the valuable 'constitnents' wererecoveredjfrom' high grade ores by; gravitymethods Tbefore grindinmillshad reached a. commercial: development;

Batera's the ores became progressively lower in grade, grinding wasemployed to unlock'mineral-"of a'" quality formerly discardedfromthetreatment'as .a tailing. In' such operations a concentration stage wasprovided in the vtreatment following each reduction in' particle; sizein the materialnnder treatment. 7 s

In-such-an operation manymachines .Were employednn relation to'thequantity ofiore treated andl'recoveriesseldom averaged; over 75%. Thenwith the advent'of froth flotation, fine grinding was. employed tounlock mineral particles of fthe type formerly lost in the tailin'gs'andrecoveries improved progressively until. today, most, l plantrecover"90%'jor overof the mineral;

Likewise under modern practice mostvplants only; provide one stage inthe circuit'where concentrates; areremoved; Ajlarge part ofithe min,-eral which initially is-freed in sizes too largevto float; consequentlyis retained in the grinding cire cuit-g andasaresu-ltinstead of having,one-1arge Darticlepermitting zit-100%" recovery; it is passed througnthemill-repeatedlyuntilfine enough to overflowthe -classifier; and'in' thetransformation becomes-thousandsof-smaller particles, someextremelyfine, thereby rendering a l'0'0-%'- recovery an impossibility: I I

Experience. thEsWOl-ld over has shown that-the majonlossesiinthestailingsioccur in the: minus 200 meslrrfractiona Consequentlyaisubstantialeliminatiom oft-production ZOfgDElMiiOl-GS; imthe-zooimeshontfinert ranges would appeanto vbe -onerneans of reducing tailingslossestl I Modern. ,practicet alsothasdeleterious-results in connection,lwith- :the treatment ofitheiganglle-l or wastei contenteThusaganguetparticle. contain? ing no mineral and too, coarse to fioatissubjected tolrepetitions ofithetgrinding action until it, too,is'redu'ced'tota' state of fineness sufli cient to pass theclassifieroverfiow':

Since the grinding stageisthe most-costly item in the operatiom ofconcentrating m-ills; it is apparent-=- that all eft'ort directed atareduction" of eral content of ores inlarger-and fewer particles than:haslbeen possiblewith methods heretofore Anotherobject of theinventionis to provide a simple, economicaljand eflicient; method ofgrindingthelmiddlingscontent'of an'ore in closed circuit.

A further object ofithe invention is to substantiallyeliminatesIimecondition's in the material under; treatment-in; anoperation utilizing closed circuit; grinding sofrthemiddlings contentof-an ore-so as to increaseu rinding. efficiency.

other-objects-ureside in the provision of: novel stepsv anditreatmentsall' of which will appear more fully iILthEzCO'llI'SGfOf thefollowingdescription-.-

The -presentrinvention has. been designed for use imawidevariety. oftreatments and; in fact, is generally applicable wherever frothflotation has been: employed-to concentrate: one or" more constituentmineralsiof a given ore. Likewise the invention is beneficial :bothintreatments where a; single mineral islseparatedf from gangue or wasteconstituents and other treatments whereintwo or moreaminerals; aredifferentiallyseparated ands recovered-as concentrates. separate fromthe gangue constituents.

In= practicingthe-present invention, the classification stage isutilizedfto obtain a separation of constituent of theore as well as tograde thematerial according'to siZeS-.:. In so doing, it is possibletoutilize a: seriesf "differential separations at eaclr 01% which:alvaluabie constituent is recovered separately-from waste matter orother matter .requiring'ladditional' treatmentbefore separation of itsmineralcontent can be effected.v

- Before undertaking: a; complete description of theoperating'procedure, it'will 'be advisable to define: certain-termsemployed: in the specification, "to distinguish .from' other meaningssometimes applied. Wherethe expressions finished ,product 'ors"finishedmaterialtare used, theylare intendew to designate; a-Mproduct groundfineenouglr-to overflow; present type classifiers and pass to subsequenttreatment; The term gangue as-used in the. specification is intended todesignate any-waste productwhich is barren of mineral.

To afford a better understanding of. the practice of the invention,reference-"is made tothe accompanyingdrawings, in which Fig. 1 is a fiowsheet in-whichatypical milling circuit has beenillustr'atediembodying:the; features of the pres-v ent invention; and,Fig: 2iis -asection'taken'along thedine 2- -32; Fig: 1':

3 In the circuit illustrated, the grinding circuit in addition to theclassification stage l and grinding stage 4, has concentrating stages 2and 3 respectively, the details of which will be fully set ficationoperation which may consist of a single,

duplex classifier or a series of individual classifiers arranged ashereinafter described.

The classification stage of the present invention is utilized to producethree products, namely, (a) finished product, (b) a mixture of coarse fre mineral with la er middlin 5 articles, and

e rg g p tice without reducing the capacity of the circuit (a) a mixtureof fine middlings with coarse angue.

While a variety of apparatus may be utilized to produce the desiredproducts at the classification stage, it will be essentially the same infunction as apparatus of the type disclosed in my co-pending applicationSerial No. 289,466 now Patent No. 2,315,536, granted April 6, 1943,which is particularly suited for this purpose. Features disclosed butnot claimed herein relating to the classification step per se, have beendescribed and claimed in the aforesaid application.

The ball mill discharge constituting the classifier feed enters the tankI at the side through a feed box l2, the discharge of which passesthrough an opening 13, and through the provision of hindered settlingmeans here illustrated as spigot compartments la and Im, the portion ofsaid feed constituting finished material is caused to pass directlyacross the classifier to its overfiow lb the discharge of which passesthrough an opening l4. At the same time the hindered settling action isutilized to induce a differential settling of the other feedconstituents, with the result that coarse free mineral mixed with stilllarger sized middlings particles are settled in one zone, while finermiddlings particles together with coarser gangue constituents aresettled in a secand zone. This hindered settling action may be producedeither by introducing gas under pressure through a suitablevalve-controlled conduit as shown at It in compartment l a, or waterunder pressure through a suitable valve-controlled conduit as shown at Hin compartment Ir.

Each of the graded settled products is separately removed from theclassification stage by suitable conveyor mechanism |5a or I511, witheach product after removal from the unitary hydraulic classificationtreatment stage being subjected to a separation with only one of theproducts so separated returning to the mill for regrinding.

To this end, the mixture of coarse .free mineral and larger middlingsparticles preferably is subjected to a jigging treatment as indicated at2, after which the doarse middling so separated is mixed with theentering feed to the mill 4, while the coarse mineral concentrate iscollected as a product for smelting.

Likewise, the rake product of classification comprising fine middlingsmixed with coarse gangue preferably is subjected to separation by frothflotation as indicated at 3, with the froth concentrate comprisingsubstantially all of the mineral content being mixed with the enteringfeed to the mill for further unlocking, while the residue productcomprising essentially coarse gangues is passed to waste.

Thus, the closed circuit grinding of the present invention difiersessentially from prior practice in that substantially all finishedproduct, coarse free mineral and barren gangue are eliminated in onepass through the classifier while the product returned to the mill forregrinding consists almost entirely of middlings requiring furtherunlocking before separation of constituents can be attained,

By so doing, the useful work of the grinding mill is limited to aninitial reduction of oversize and further unlocking of mineral-bearingmaterial only. The elimination of finished product in the first passthrough the classifier prevents sliming to any appreciable degree.

As a consequence of these economies in operat'ion,.the capacity of thegrinding mill may be reduced from that required in conventional pracandin this way conomies in initial cost and operating cost are efi'ected.

The product overflowing the classifier at lb is then taken as thefeed'of the flotation circuit, here represented as a plurality ofrougher cleaner cells. The concentrate of this operation is then passedto a thickener 6, and the collected solids of the thickening operationare filtered as shown at l and are then ready to go to a smelter 8 whichmay or may not be associated with the milling operation.

The concentrate of the jig 2 also is taken to smelter 8 and whereshipment is necessary to get these concentrated minerals from the millto the smelter, they may be mixed for convenience in transportation.

The elimination of slimes from the feed to flotation has beneficialeifects of a pronounced character. In the first place, a granularflotation concentrate requires smaller thickener and filter areas.Likewise, a granular filter cake contains less moisture than a slimyone, and where such products must be shipped to a smelter, a substantialsavings in shipping expense results.

Furthermore, a granular concentrate has a much lower dust loss in thesmelting operation and consequently a further economic gain results fromthe treatment of the present invention.

From the foregoin description of the practice of the present invention,it will be understood that the purpose of the initial separations is tofree the product recirculating through the grinding mill ofsubstantially all its barren gangue content, and all its free mineral,irrespective of size.

To this end, the finished product containing both free mineral andgangue, in sizes fine enough to pass the classifier overflow, isdischarged from the grinding circuit without re circulation and is thenin ideal condition for the subsequent flotation separation as the slimeformation factor has been eliminated.

Likewise, the graded settling of the oversize products permits isolationof the middling by screen or gravity separation methods. Consequently,the material returning for regrinding is almost exclusivelymineral-bearing and. for the most part Of a size permitting furtherreduction without producing slimes.

For example, in the operation a given middlings product of the mill maybe substantially oversized and comprise gangue and 10% mineral. Whenreturned and further reduced, it is split into one oversize piece ofgangue, and two smaller size pieces, one being classed as coarsemiddling and the other fine eeeeigcsa risiddlrnga Maltese-arrayco-nrprls'efirouglily m /s mian Wamiii eral; 1 I I At this stage -the'speclfie grayitiesiofi these iniddiihgsi coastituents readilyiidentifies them ir'omitlia gangue and from tlie-mineral.

This condition permits making the-' tail ing at a substantiali'oarserhrindithamis possiblein I :cticel With tlie"process"I -ha-yedeveloped it is possible; to isol'ate themiddli-ng atethe screen size atwhich tlrel r'a-ti'c'i off. mineral to gangue in a particle gives it aspecific gravity whicir: idritifiesiit -from' the free minerai -i on onerhanugandatheiganguezon tlierctherx While jigging has been desfgztratc.as; pres ferried: treatment: for: separatiorrl .of coarsea free mineraland larger sized middlings, it willibe understood that screening orother forms of gravity separation may be substituted where desired.

Similarly, froth flotation has been designated as the preferredtreatment for collecting the fine middlings separately from theassociated gangue, but screening or other forms of gravity separationalso may be substituted in this stage as well.

As previously suggested herein, the preferred structural arrangement atthe classification stage is a duplex-type classifier having a transversetrough or channel for the direct movement of the finished materialthrough the classification stage and out of the treatment. To insureprompt elimination of substantially all finished material, theclassifier feed preferably is subjected to hindered settling influencesbut other forms of treatment apparatus capable of attaining the sameresults may be substituted in place of the classifier hereinbeforedescribed.

As the aforesaid treatment is not dependent upon chemical change orreaction except to the extent that froth flotation or leaching isrecommended, it will be understood that all ores containing mineralwhich can be recovered on a commercial scale by ore dressing methods maybe treated efiectively by the methods of the present invention.

It is to be understood further that the invention is in no sense limitedto the arrangement of treatment steps set forth in the accompanying flowsheet which is intended merely as a typical example, but rather embodiesas essentials the elimination of substantially all ore constituentsother than middlings in the product returned to the grinding circuit.

In the middlings flotation treatment the product treated is ofrelatively coarse mesh size and a large amount of reagent is required tocollect all of the particles of varying mineral content in the frothconcentrate. However, with slimes eliminated from the feed as described,the operation in this treatment becomes quite efiicient, whereasif theslimes were permitted to remain in the feed the reagent would carry upall fines, both middlings and gangue, resulting in further sliming andwasteful grinding in the mill.

For this reason, flotation is recommended for the treatment of themixture of fine middlings and coarse gangue. But as pointed outhereinbefore, a variety of treatments may be employed at this and otherstages within the spirit and scope of the invention as defined in thehereunto appended claims.

More specifically, certain operations will involve leaching operationsfor the recovery of mineral in place of the froth flotationconcentration 'hereinbefore described. The present process may utilizeleaching ateeitheriofithestagesdesignated 3 or Ssinthe fiow sheet.

However best results in leaching operations will? be attained when thesecond commitment o.2the-.classi fier is used f61 .Sa|IldS=-S1im-.separationsaridpacli product of tliislseparation thereafter is subjectedto a difirent type of'leaching.

In such an operation, the=slimes or finished materialswill be passedfrom the-igrinding stageeto leaching with agitationatqaastage actingasan alternate forthe. flotation stage 5 of the fiow sheet. The sandsso'separatewwill'pass to a still eechmgoperatiomservinaasanalternatez-rorzthe sta e: 31: illustrated.Otherwise the treatment steps will be substantially asz hereinberore:des

cribed.

What I claim and desire tov secure by Letters Patent is:

1. In an ore dressing circuit, the steps of grinding an ore in a mill toproduce a product containing some coarse free mineral, passing thegrinding mill discharge containing granular material with only a minorquantity of slimes sizes directly into a two-stage classificationseparation, inclusive of a hindered settling treatment in which finishedmaterial, inclusive of slimes, is

maintained at or near the surface and eliminated directly from thecircuit, controlling the feed of hydraulic water to the hinderedsettling zone to produce two settled products, one consistingpredominantly of coarse middlings together with some free mineral andthe second consisting predominantly of fine middlings together withcoarse gangue, elevating said segregated products through the liquid ofthe classification stage so that the discharge products are essentiallyde- Watered, subjecting the coarse middlings and free mineral to aconcentration treatment for the recovery of the free mineral, subjectingthe fine middlings and coarse gangue to a difierent concentrationtreatment for the collection of the fine middlings and elimination ofthe coarse gangue from said circuit, and subjecting only the saidmiddlings products so separated to regrinding in said circuit.

2. An ore dressing circuit of the character described in claim 1 whereinthe concentration treatment to which the coarse middlings and freeminerals are subjected comprises a jigging treatment.

3. An ore dressing circuit of the character described in claim 1 whereinthe concentration treatment to which the fine middlings and coarsegangue are subjected comprises a gravity concentrating treatment.

HARRY L. McNEILL.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,710,522 Trumbo Apr. 23, 19291,930,709 DeLore Oct. 17, 1933 1,939,119 Holt Dec. 12, 1933 1,955,039Weinig Apr. 17, 1934 2,065,722 Munro Dec. 29, 1936 2,106,290 Akins Jan.25, 1938 2,107,289 Denuy Feb. 8, 1938 2,147,234 Bird Feb. 14, 19392,156,245 Mead Apr. 25, 1939 2,163,285 McNeill June 20, 1939 2,180,574Wood Nov. 12, 1939 (Other references on following page) 7 UNITED STATESPATENTS Number Name Date 2,194,139 Davis Mar. 19, 1940 2,312,865 BirdMar. 2, 1943 2,330,158 Tartaron Sept. 21, 1943 2,336,854 Ferris Dec. 14,1943 FOREIGN PATENTS Number Country Date 294,210 Great Britain Nov. 12,1929 OTHER REFERENCES Denver Equipment Index, published by DenverEquipment 00., Denver, Colorado, 1936, pages 82, 83, 84, 100, 104, 106.

Milling Meth'ods, 1934, Transactions of American Institute of Mining andMetallurgical Engineers, volume 112, published by American Institute ofMining and Metallurgical Engineers, New York, 1935, pages 563, 564, 579,580, 581, 707, 708, 709, 711, 712, 715, 867, 868, 869, 870, 871, 889,890, 891, 394.

Milling Methods at Concentrator of Miami Copper Company, Bureau ofMines, I. C. 6573, Apr. 1932, Figures 1, 2, 3, pages 2, 3.

Truscott, Text Book of Ore Dressing, 1923,-page 297.

Reports of Investigation, Bureau of Mines, No. 3328, pages 5 to 35 and.9 unnumbered pages; No. 2618, pages 1 to 6,

Gaudin, Principles of Mineral Dressing, page 413.

